Aluminum is produced conventionally by the electrolysis of alumina dissolved in cryolite-based (usually as NaF plus AlF3) molten electrolytes at temperatures between about 900° C. and 1000° C.; the process is known as the Hall-Heroult process. A Hall-Heroult reduction cell/“pot” typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon that contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate that forms the cell bottom floor. The carbon lining and cathode substrate have a useful life of three to eight years, or even less under adverse conditions. In general carbon anodes are consumed with evolution of carbon oxide gas, as bubbles and the like.
The consumption of carbon anodes in molten electrolyte is shown in FIG. 6a of U.S. Pat. No. 2,480,474 (Johnson). Anodes are at least partially submerged in the bath and those anodes as well as their support structures are replaced regularly once consumed. Alumina is fed into the bath during cell operation and it is important to have good alumina dissolution. The anode gas bubbles can be used to create a turbulence in the alumina feeding zone to reduce alumina agglomeration. It is important to create a good turbulence by anode gas bubbles to the extent favorable to increase alumina dissolution.
Traditional technology relied on natural flow of gases from under the carbon anodes curing the aluminum reduction process, but this delayed gas bubble removal and decreased efficiencies and aluminum production.
This presence and build up of gas generated during electrolysis has been a continuing problem in the industry and a cause of high energy requirements, and to efficiently operate the electrolysis cells, the electrodes must be properly designed. Dell et al., in U.S. Pat. No. 3,822,195, taught bipolar anodes having channels (nine shown in FIG. 2) on their bottom between a downward gas dam along three sides of the anode marginal ledge communicating with a lateral connecting channel. This patent relates to the production of aluminum electrolytically from a metal chloride dissolved in molten solvent. The described design does not apply to the Hall cell process since the anodes in the chloride system are non-consumable.
Use of single and multiple bottom anode tracks, across the entire anode bottom, to improve gas release in aluminum processing has also been reported in Light Metals, “How to Obtain Open Feeder Holes by Installing Anodes with Tracks” B. P. Moxnes et al., Edited by B. Welch, The Minerals, Metals & Materials Society, 1998, pp 247-255. There for a 141 cm anode, a track width of 2 cm was suggested, as tracks less than 1 cm did not drain gas properly, while the maximum height suggested was 16 cm.
U.S. Pat. No. 5,330,631 (Juric et al.) relates to an aluminum smelting cell and describes anodes with downwardly extending peaks, V shaped profiles and angularly positioned inward protrusions each having three sides to achieve desired electrolyte bath flow and controlled bubble release. Somewhat similarly, de Nora in U.S. Pat. No. 5,683,559 teaches grooves in cathodes said to improve gas circulation, as well as outwardly sloped V shaped anodes for electro winning of aluminum.
All of the designs pose a number of problems. Natural flow decreases efficiencies, continuous slots disrupt the metal bath interface between the anode and cell sidewall causing loss of current efficiency, and a large number of slots causes problems of extensive machining and loss of carbon.
What is needed is a carbon anode design that facilitates gas bubble movement rapidly to the centerline of the reduction cell to expedite dissolution of alumina, including alumina fines that typically float on top of the bath and are slow to dissolve. At the same time, the carbon anode design should allow the pots to operate at a lower pot noise and reduced pot voltage and therefore lower power consumption and higher current efficiency.
It is therefore one of the main objects of this invention to minimize machining and expense in carbon anode manufacture, to facilitate dissolution of alumina fines and increase cell life, and to reduce anode gas bubble voltage causing less energy consumption. The term “bubble” as used herein is defined to mean and include any gas entrapment, whatever its shape. Initially small discrete round or oval bubbles do form; but they rapidly coalesce to form a flattened sheet-like configuration until released. Then new discrete round or oval bubbles start to form again on the anode surface.